Fiber reinforced composite article, fiber member, and method for making

ABSTRACT

A fiber reinforced composite article, comprising a matrix and reinforcing fibers and subjected during operation concurrently to a plurality of temperatures and stresses, varying between a plurality of regions of the article, experiences different stresses concurrently in different regions of the article. The article is provided with fiber reinforcement of a strength in each region greater than the stress experienced in that region. Such fiber reinforcement is provided through a member for inclusion in the matrix, for example in the form of at least one of a fabric, weave, braid, lay-up, etc. One form of such an article for use at relatively high temperatures is a turbine engine component, for example a gas turbine engine exhaust flap. Another form of such an article is a gas turbine engine blading component.

BACKGROUND OF THE INVENTION

This invention relates to a member comprising fibers, to a compositearticle reinforced with fibers, and to a method for making such memberand article. More particularly, in one form, it relates to a fiberreinforced composite article that, during service operation, experiencesconcurrently a plurality of temperatures and stresses varying between aplurality of regions of the article.

In power generation apparatus, for example a gas turbine engine, widevariations in operating temperatures are experienced from inlet toexhaust, as well as across or within individual components of such anengine. An example of such thermal variation has been observed withinthe engine exhaust system across or within regions of panels such asexhaust flaps. Another example is a turbine engine blading component,such as a blade, vane, strut etc., having a surface or skin region thatexperiences a temperature greater than does reinforcing or strengtheningspars in the component.

Such high thermal gradients across or within such an article orcomponent or within a region, sometimes called a hot spot, of thearticle has generated relatively high thermal strains sufficient tolimit the operating capability of the article. For example, cooler edgeregions of a gas turbine engine exhaust flap, made of a high temperatureoxide fiber reinforced ceramic matrix composite material, existingconcurrently with a higher temperature region within or away from theedge regions, have been observed to develop cracks that can decrease theoperating life of the article. Such degradation is believed to resultfrom the inability of the reinforced structure of an article, madegenerally from a single kind of material, to compensate for differentstress levels developed from a stress conflict between differenttemperature regions. The reinforced matrix of the composite includingreinforcing fibers, for example of a single kind, alone does not havethe capability to withstand or compensate for such wide strainvariations across the operating temperature range of the article.

BRIEF SUMMARY OF THE INVENTION

The present invention, in one form, provides a fiber reinforcedcomposite article comprising a matrix and reinforcing fibers. Duringoperation, the article experiences concurrently in the article aplurality of operating temperatures varying between a plurality ofregions of the article. For example, operating temperatures can varywithin a total temperature range of about 900–2000° F. In one form ofoperation, a first region of the article experiences a firsttemperature, for example of about 1800° F. in the range of about1600–2000° F., that develops a first stress in that first region of thearticle. Concurrently in operation with the first region, a secondregion of the article experiences a second temperature less than thefirst temperature, for example of about 1100° F. in the range of about900–1300° F., that develops a second stress in that second region of thearticle greater than the first stress. Such difference in stress betweenregions of a fiber reinforced composite article has been observed toresult in cracking in higher stressed, lower temperature region, forexample at an edge portion of an article.

A form of the present invention provides, for a plurality of regions ofan article, strength levels of reinforcing fibers adequate to compensatefor the operating stress level in its respective region. The fibersappropriately are selected for each region in making an article or areinforcing member, to provide a strength level greater than therespective stress experienced at operating temperature in that region.In part, as will be described below, such strength levels are based onuse of fibers in a matrix having different coefficients of thermalexpansion (CTE), sometimes referred to as “α”, determined at theintended operating temperature. In one form, the selection of fibers toprovide appropriate strength levels in the regions is based on adifference in thermal stress levels from the product of the followingfor each region: a ratio of each respective area of a region to thetotal area of an article portion, the elastic modulus of the fiberreinforced matrix, the CTE of such reinforced matrix, and thetemperature at which each region operates. In that form of theinvention, such product of the first, relatively hotter, region is lessthan such product for the second, relatively cooler region. One form ofsuch a relationship, to be referred to hereinafter as the“relationship”, is A₁E₁α₁T₁−A₂E₂α₂T₂<S_(2,) in which, as identifiedabove, respectively for the regions 1 and 2, A is the area ratio of aregion to a total area of the regions, E is the elastic modulus of thefiber reinforced matrix, α is the CTE of the fiber reinforced matrix atthe operating temperature in ° F., T is the operating temperature in °F., and S is the strength of a relatively cooler region. In thisexample, the second region, region 2, has a second strength greater thanthe first strength in the first region.

Another form of the invention is a member in the form of a mixture ofsuch first and second fibers, for example as a fabric, weave, braid,lay-up, etc., used to make a fiber reinforced composite article. Stillother forms are methods of making such a member or making such anarticle, comprising selecting the first and second fibers and theirrelative mixture and location in the member or article as a function oftheir intended operating conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a panel representing one form ofarticle provided according to the present invention, in which operatingtemperature varies from a region within the panel outwardly toward thepanel edge regions.

FIG. 2 is a diagrammatic view of a panel representing another form ofthe present invention, in which operating temperature variessubstantially from a first end region to a second end region.

FIG. 3 is a comparison of variation in coefficient of thermal expansionacross a range of temperatures for two different fiber reinforced matrixcomposite systems.

FIG. 4 is a diagrammatic view of a member in still another form of thepresent invention: in the form of woven fabric comprising, in selectedareas, a single or a mixture of first and second fibers, according tothe present invention, for use in making a fiber reinforced compositearticle.

DETAILED DESCRIPTION OF THE INVENTION

Currently, one form of gas turbine engine exhaust flap, convenientlyrepresented generally in the form of a panel, is made of a ceramicmatrix reinforced generally uniformly throughout with a single kind ofhigh temperature oxide fiber. One form of such an exhaust flap includesan aluminosilicate matrix, made from commercially available SM8sub-micron alumina powder and silica. The matrix is reinforced with anoxide fiber nominally comprising, by weight, 85% alumina and 15% silica,and commercially available as Nextel 720 roving material. During onetype of service operation, the exhaust flap experienced a firsttemperature ranging from about 1600–2000° F. in at least one region awayfrom peripheral edge regions, within the panel, to a second temperatureless than the first temperature and in the range of about 900–1300° F.at peripheral regions of the flap. It was observed that such temperaturevariation in the flap caused cracking at peripheral edges resulting fromexcessive strain of the composite at such cooler points, whilemaintaining the integrity of the structure in the hotter region.

One diagrammatic embodiment of a panel intended to experience the typeof service operation described above is shown in the diagrammatic viewof FIG. 1. The panel shown generally at 10 represents, diagrammatically,an article such as a gas turbine engine exhaust flap. During serviceoperation of the panel, the at least one first region 12, away fromperipheral edge or second region 14 is shown to be generally discretefrom edge region 14 within the panel. First region 12 experiences afirst temperature greater than a second temperature experienced by edgeregion 14.

One form of the present invention avoids generation of excessive,destructive strain in the panel by disposing in each of the first andsecond regions 12 and 14 different high temperature reinforcing fibers,or a mixture or blend of a plurality of high temperature reinforcingfibers. Each fiber or mixture of fibers has a different coefficient ofexpansion (CTE) at the temperature at which it operates, to provide,with the matrix, the above described “relationship”. Each of thedifferent fibers in the respective regions, or the mixture or blend, istailored or selected between regions to provide in a region a strengthlevel adequate to resist in the article the generation of excessivestrain sufficient to cause degradation in the article structure acrossthe entire operating temperature range. That embodiment of the inventionuses, in a cooler region, a single kind of fiber or a mixture of fibershaving, in the single kind of fiber or in the mixture of fibers,relatively high thermal expansion characteristics to yield appropriatelyhigher thermal strain. Concurrently, that embodiment uses, in arelatively hotter region, fibers having, in the single kind of fiber orin the mixture of fibers, a relatively lower thermal expansioncharacteristic. The result, through the “relationship”, is reduction inthermal strain, including, in some forms, resistance to biaxial thermalstrains, in the cooler region that previously has resulted in crackingfrom excessive thermal strain. Examples of high temperature fibers usedin forms of the present invention include, but are not limited to, atleast one or a combination of alumina, silica, glass, graphite, carbon,carbides, tungsten, and boron.

Another diagrammatic embodiment of a panel 10, intended to experienceconcurrently a plurality of temperatures in regions of the articleduring operation, is shown in the diagrammatic view of FIG. 2. In thatembodiment, a first end region 16 experiences a first temperaturegreater than a second end region 18, spaced apart from first end region16.

One embodiment of the present invention for reinforcing an aluminamatrix used two kinds of fibers based on alumina in the “relationship”.A first of the plurality of fibers, in a method for making a reinforcingmember for use in making a fiber reinforced matrix composite article,selected predominantly for a first, higher temperature region of about1650° F., representative of the range of about 1600–2000° F., had afirst CTE of about 3.37×10⁻⁶/° F. at that temperature of 1650° F. Inthis example, the first fiber predominantly for the higher temperaturewas the above described Nextel 720 ceramic fiber roving. A second of theplurality of fibers in this embodiment, used alone or in combinationwith other fibers for a second, lower or cooler temperature region ofabout 1100° F., representative of the range of about 900–1300° F., was acommercially available fiber Nextel 610 ceramic roving substantially ofalumina, with a second CTE of about 3.93×10⁻⁶/° F. at the secondtemperature, and greater than the first CTE at the first temperature.

The above-identified fibers were used in making one series of articlespecimens for evaluation of fiber reinforced ceramic matrix compositeforms of the present invention. The matrix an ultra pure, sub-micronsize ceramic of substantially pure alumina, marketed by BaikowskiInternational Corporation as SM8 material, mixed in a silica binderprovided by the thermal decomposition of a polymer including silicon.The matrix comprised, by weight, about 80% alumina and about 20% silica.The specimens were woven fabric reinforced composites including varioussingle or hybrid reinforcing mixtures of the above-identified Nextel720, (identified below as “N7”) and Nextel 610, (identified below as“N6”) ceramic rovings. In this specific series of evaluations, thefollowing values of the above-described “relationship” were used for thespecimens:

-   -   A₁=0.66; A₂=0.33    -   T₁=1650° F.; T₂=1100° F.    -   E_(N7): @T₁=11×10⁶; @T₂=10.4×10⁶    -   E_(N6): T₁=12×10⁶; @T₂=11.8×10⁶    -   α_(N7): @T₁=3.37×10⁻⁶; @T₂=3.03×10⁻⁶    -   α_(N6): T₁+4.45×10⁻⁶; @T₂3.93×10⁻⁶    -   S_(N7)@T₂=21000 pounds per square inch (psi)    -   S_(N6)@T₂=35000 psi

In one example, an article specimen had all Nextel 720 rovings as thereinforcing fiber. Using the above values, the difference of the twoproducts in the “relationship” calculated to be a stress of about 28,900psi. Because the stress value 28,900 psi is greater than the strength ofabout 21,000 psi of Nextel 720 material in the region at 1100° F., theregion of such a fiber reinforced composite article at about 1100° F.can degrade and crack from the excessive stress.

In another example, an article specimen had all Nextel 610 rovings asthe reinforcing fiber. Using the above values, the difference of the twoproducts in the “relationship” calculated to be a stress of about 41,300psi. Because the stress value 41,300 psi is greater than the strength ofabout 35,000 psi of Nextel 610 material in the region at about 1100° F.,the region of such a fiber reinforced composite article at about 1100°F. can degrade and crack from excessive stress.

In still another example, this one according to a form of the presentinvention, an article specimen had all Nextel 720 rovings in the hotterregion 1 at 1650° F. and all Nextel 610 rovings in the cooler region 2at 1100° F. Using the above values, the difference in the two productsof the “relationship” calculated to be a stress of about 23,500 psi.Because the operating stress value 23,500 psi is less than the strengthof about 35,000 psi in the region at 1100° F., the strength of the fiberin that region would exceed the operating stress. Therefore according toa form of the present invention, degradation or cracking in that regionof an article would not occur, while the integrity of the hotter regionat 1650° F. would be maintained using Nextel 720 rovings in that hotterregion.

FIG. 3 is a graphical comparison of coefficients of thermal expansion atdifferent temperatures of the above-identified Nextel 610 and Nextel 720alumina-based ceramic rovings in an alumina-based matrix includingsilica, of the type described above. The data in FIG. 3 for those twotypes of fiber reinforced matrix composites represent the type of datathat can be used in connection with the present invention in the“relationship” to select the type and amounts of fibers for theplurality of regions of an article, operating at different temperatures.

As can be appreciated, mixtures or blends of fibers concurrently fordifferent regions of an article can be selected to be of a single type,and/or of various mixtures or blends as are appropriate to provide,according to embodiments of the present invention, the strength in aregion greater than that region's operating stress. In one preferredembodiment of the present invention for a high temperature gas turbineengine component, fibers are selected for or included in a region of amatrix in the range of about 20–70% by volume. At least about 20 vol. %fiber is required to provide adequate reinforcement. A composite withgreater than about 70 vol. % fiber includes insufficient matrix for theintegrity of the composite.

One example of a member for fiber reinforcement provided in accordancewith a form of the present invention and having concurrently a pluralityof different combinations of fibers is shown in the diagrammatic view ofFIG. 4. The member, shown diagrammatically generally at 20 in FIG. 4, isa woven fabric including various regions of fiber combinations for useas fiber reinforcement in a fiber reinforced composite article. In oneform of manufacture of a fiber reinforced matrix composite article, aplurality of members 20 are disposed in a stack, for example maintainingthe same relative position shown in FIG. 4, as the fiber reinforcementfor a matrix.

Member 20, representing different combinations of fibers that can beselected, and if necessary combined, appropriately for intendedoperating conditions, was woven with different fiber combinations, in apattern repeated in the fabric. Then the fabric was cut in desiredshapes for stacking. Regions 22, representing a hottest operating regionof a member, included all Nextel 720 rovings in both the warp and filldirections. Regions 24, representing a coolest operating region of amember, included all Nextel 610 rovings in both the warp and filldirections. Regions 26 and 28 represented regions with operatingtemperatures between those of regions 22 and 24. Region 26 included allNextel 720 rovings in the warp direction and all Nextel 610 rovings inthe fill direction. Region 28 included all Nextel 610 rovings in thewarp direction and all Nextel 720 rovings in the fill direction. Aplurality of this embodiment of a member of the present invention wasarranged in a stack to provide the fiber reinforcement for a fiberreinforced alumina matrix composite. The composite included about 45vol. % fibers, with the balance essentially alumina matrix, within thepreferred range of about 20–70 vol. % fibers.

The present invention has been described, in various general embodimentsand forms, in connection with specific examples and combinations.However, it should be understood that these are intended to be typicalof rather than in any way limiting on the scope of the presentinvention. Those skilled in the various arts associated with thisinvention will understand that it is capable of variations, combinationsand modifications without departing from the scope of the appendedclaims.

1. A fiber reinforced composite article comprising a first surface, asecond surface opposed to and spaced apart from the first surface, and amatrix and reinforcing fibers therebetween wherein: the articlecomprises a plurality of discrete regions each extending completelythrough the first and second surfaces and the matrix of the articletherebetween; a first region of the plurality of regions of the articleduring operation use subjected to a first temperature and a firststress, and including first fibers having a first strength greater thanthe first stress; and, a second region of the plurality of regions ofthe article during operation use subjected to a second temperature lessthan the first temperature and a second stress greater than the firststress, and including second fibers having a second strength greaterthan the second stress.
 2. The article of claim 1 in which: the firstfibers have a first coefficient of thermal expansion (CTE) at the firsttemperature; and, the second fibers have a second CTE at the secondtemperature greater than the first CTE at the first temperature.
 3. Thearticle of claim 2 in which the second strength of the second region isgreater than the difference between the first stress and the secondstress as determined by the relationship:A ₁ E ₁α₁ T ₁ −A ₂ E ₂α₂ T ₂ <S _(2,) in which, respectively for thefirst region (1) and the second region (2): A is the area ratio of aregion area to a total area of the regions, E is the elastic modulus ofthe fiber reinforced matrix, α is the CTE of the fiber reinforced matrixat the operating temperature in ° F., T is the operating temperature in° F., and S is the strength of the second region.
 4. The article ofclaim 3 in which the first and second fibers are in at least one formselected from the group consisting of fabric, weave, braid, and lay-up.5. The article of claim 3 in which: the first temperature is in therange of about 1600–2000° F.; and, the second temperature is in therange of about 900–1300° F.
 6. The article of claim 5 in which thematrix is ceramic based on alumina.
 7. The article of claim 5 in whichthe fibers are included in the range of about 20–70 volume %.
 8. Thearticle of claim 6 in which the first and second fibers are based onalumina.
 9. The article of claim 6 in which the matrix includes silica.10. The article of claim 5 in the form of a turbine engine article inwhich: the matrix is a ceramic; and, the first fibers and the secondfibers are high temperature fibers made from at least one materialselected from the group consisting of alumina, silica, glass, graphite,carbon, carbides, tungsten, boron, and their mixtures.
 11. The articleof claim 9 in the form of a gas turbine engine exhaust flap in which thefibers are included in each region in the range of about 20–70 volume %.12. The article of claim 4 in the form of a gas turbine engine bladingcomponent in which the fibers are included in each region in the rangeof about 20–70 volume %.
 13. A member comprising reinforcing fibers forreinforcement of a fiber reinforced composite article, the membercomprising a first surface, a second surface opposed to and spaced apartfrom the first surface, and a matrix and the reinforcing fiberstherebetween wherein: the member comprises a plurality of discreteregions each extending completely through the first and second surfacesand the matrix of the member therebetween; a first region of theplurality of the member during operation use subjected to a firsttemperature and a first stress, and including first fibers having afirst strength greater than the first stress; and, a second region ofthe plurality of the member during operation use subjected to a secondtemperature less than the first temperature and a second stress greaterthan the first stress, and including second fibers having a secondstrength greater than the second stress.
 14. The member of claim 13 inwhich: the first fibers have a first coefficient of thermal expansion(CTE) at the first temperature; and, the second fibers have a second CTEat the second temperature greater than the first CTE at the firsttemperature.
 15. The member of claim 14 in which the second strength ofthe second region of the article is greater than the difference betweenthe first stress and the second stress as determined by therelationship:A ₁ E ₁α₁ T ₁ −A ₂ E ₂α₂ T ₂ <S _(2,) in which, respectively for thefirst region (1) and the second region (2): A is the area ratio of aregion area to a total area of the regions, E is the elastic modulus ofthe fiber reinforced matrix, α is the CTE of the fiber reinforced matrixat the operating temperature in ° F., T is the operating temperature in° F., and S is the strength of the second region.
 16. The member ofclaim 14 in at least one form selected from the group consisting offabric, weave, braid, and lay-up.
 17. The member of claim 15 in which:the first temperature is in the range of about 1600–2000° F.; and, thesecond temperature is in the range of about 900–1300° F.
 18. The memberof claim 17 in which the first and second fibers are based on alumina.19. The article of claim 1 in which: the first region comprises a firststack of first fiber woven fabric shapes, the first stack extendingcompletely through the first region, the first woven fabric shapescomprising a first combination of reinforcing fibers; and, the secondregion comprises a second stack of second fiber woven fabric shapes, thesecond stack extending completely through the second region, the secondwoven fabric shapes comprising a second combination of reinforcingfibers having a coefficient of thermal expansion (CTE) during operationof the article different from the first combination.
 20. The article ofclaim 19 in which the first combination of reinforcing fibers of thefirst woven fabric shape for the first stack and the second combinationof reinforcing fibers of the second woven fabric shape for the secondstack each are included in a pattern repeated in a fabric member, aplurality of the fabric members disposed in a composite article stackthat maintains the same relative position of the pattern completelythrough the composite article stack to provide the first and secondregions.
 21. The member of claim 13 in which: the first region comprisesa first stack of first fiber woven fabric shapes, the first stackextending completely through the first region, the first woven fabricshapes comprising a first combination of reinforcing fibers; and, thesecond region comprises a second stack of second fiber woven fabricshapes, the second stack extending completely through the second region,the second woven fabric shapes comprising a second combination ofreinforcing fibers having a coefficient of thermal expansion (CTE)during operation of the article different from the first combination.22. The member of claim 21 in which the first combination of reinforcingfibers of the first woven fabric shape for the first stack and thesecond combination of reinforcing fibers of the second woven fabricshape for the second stack each are included in a pattern in a fabricmember, a plurality of the fabric members disposed in a compositearticle stack that maintains the same relative position of the patterncompletely through the composite article stack to provide the first andsecond regions.